In recent years, there is a globally increasing concern about air pollution due to industrial emission of harmful oxides of sulfur and carbon. Accordingly, an allowable amount of the emission of the one or more pollutants has been regulated and the regulation tends to become stricter.
Treatment of nitrogen oxides is particularly required since the nitrogen based oxides not only is harmful as itself but also produces more harmful secondary pollutants by photochemical reaction in an atmosphere.
Among the conventional catalysts used in the treatment of an exhaust gas containing nitrogen based oxide, there has been reported a catalyst in which metal such as Cu is sulfur-oxidized and impregnated on a silica support as a catalyst having a high N2 reduction ratio (Japanese Patent Publication No. H8-173766).
But, this catalyst has a disadvantage that a volatile sulfuric compound is required to be added to an exhaust gas in order to maintain the metal in a sulfur-oxidized state during the reaction. There has also been a catalyst which impregnates vanadium oxide, tungsten oxide and palladium on titania and/or titania silica (Japanese Patent Publication No. 2001-293480).
But, there is a problem that an activity of the catalyst is sharply lowered at a high temperature (above 400° C.), and the use of this catalyst is regulated due to toxicity of the vanadium.
A catalyst using zeolite as a support or mainly containing zeolite substituted with metal having an activity to nitrogen oxide is fabricated and used in a form of a sphere, a pillar, a granule or a honeycomb. However, the zeolite powder itself has insignificant plasticity and sinterability.
A technology of fabricating a molded body which is not broken by crack upon fabrication of a zeolite support or a zeolite catalyst and has a high specific surface area is therefore very important and has been actively studied.
In one example, an inorganic binder such as natural clay, bentonite, kaolin, colloidal silica or a cellulose based organic binder is added (U.S. Pat. No. 5,116,586), but there is a disadvantage that a performance of the catalyst is notably reduced since it is required to add relatively large amount of inorganic binder and a content of the zeolite in a fabricated zeolite support or zeolite catalyst is consequently lowered.
Particularly, a zeolite honeycomb type support or zeolite catalyst, which can have a high specific surface area, requires high moldability and high physical strength as compared with a sphere or pellet type having a simple structure since it has a structure in that internal pores passing through the molded body are regularly formed and is therefore difficult to be fabricated. Accordingly, a wash coated type zeolite which is fabricated by wet pulverizing zeolite and catalyst materials having an activity to nitrogen oxide and coating them on a surface of previously fabricated structural body with a honeycomb shape has been mainly used.
However, in the wash coated type zeolite, an efficiency of nitrogen oxide removal per unit volume is lower than that of the zeolite honeycomb type extrudate and the efficiency of nitrogen oxide removal is sharply reduced as a catalyst is largely damaged by erosion when used for a long time.
As a solution to the problem of forming a catalyst mainly containing the zeolite, there have been suggested a method in which zeolite is added with polysaccharide and selectively clayey mineral or inorganic fiber, molded in a honeycomb shape and then calcined at 500 to 800° C. (Japanese Patent Publication No. 19995-053208), and a method in which salt treated mesoporous zeolite is mixed with pseudo-boehmite, organic binder, plasticizer and lubricant, aged at a low temperature, molded in a honeycomb shape, dried and then calcined at a high temperature (Korean Patent Publication No. 2004-0063630).
In the method disclosed in Korean Patent Publication No. 2004-0063630, pseudo-boehmite is used as an inorganic binder like the present invention, but zeolite, inorganic binder, plasticizer and lubricant are simultaneously kneaded and molded. Therefore, shape retention and fine moldability of a molded body is still insufficient.
From numerous experiments and hard work, the present inventors have been found that gelatinization of the pseudo-boehmite used as an inorganic binder is the most important factor determining viscosity and strength (moldability of ultra fine structure) of paste allowing the fabrication of a zeolite honeycomb type extrudate with high specific surface area and an ultra fine porous structure in which a wall thickness between internal pores is 0.1 to 2 mm and the factor determining damage by crack generated upon dry and heat treatment. The present inventors have also been found that the gelatinization of the pseudo-boehmite is sensitively varied as pH, moisture content and time taken to perform the gelatinization and quality of the molded body (e.g. macro crack and damage) is varied as the step of performing the gelatinization, and thereby reached the present invention. In addition, the present inventors have been found that gelatinization in a basic state is very effective, and thereby reached the present invention.